Wiring 20 amp/15amp in a master bath.

I am working on wiring my new master bath. I have a GFCI receptacle, an air vent with a light, a light in my shower and a light over my vanity. I ran a 12/2 (20 amp) from my subpanel up to my GFCI. Will it be okay to then run 14/2 (15 amp) from the GFCI to the rest of my fixtures? I have looked at the 2012 IRC book to see how the code reads but nothing stood out as being rock solid. The only thing being rock solid was the GFCI. I have been told different things. You are probably going to ask me, why not just wire the whole room with 12/2? Again, I was told by those in my area here to do it this way.
Any help would be appreciated. Thanks in advance.

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Wiring 20 amp/15amp in a master bath.

There are lots of codes for bathrooms. However you would never down size a bathroom receptacle branch circuit to a size 14 awg from a 12 awg at a gfci. The gfci must be powered by a 20 amp branch circuit to comply witrh the code requirements. If you switch to 14 awg to power the lights then you would have to downsize the circuit to 15 amps. You cannot stay compliant with bathroom codes for receptacles if you do that. So your options are to stay with 12 awg for any continuation of the branch circuit from the gfci to power lights or fans in that same bathroom or run a separate 14 awg 15 amp branch circuit for the lighting and other equipment.. Also remember if only one bathroom is involved you cannot extend to another area outside the bathroom if you power everything in that bathroom from the single 20 amp branch circuit.

Just to add you would never downsize branch cirucit wiring from a larger awg to a smaller awg in the same branch circuit when running new branch circuit wiring for any room in a home. In general you simply do not do this no matter the situation.